Use of thin film field emitters in luminographs and image intensifiers



INVENTORS ATTORNEY William L. Roberts 8 William E. Newell.

WITNESSES @MA ma? United States Patent O USE F THIN FILM FIELD EMITTERSIN LUMINO- GRAPHS AND IMAGE INTENSIFIERS William L. Roberts,Monroeville, and William E. Newell, Penn Township, Allegheny County,Pa., assignors t0 Westinghouse Electric Corporation, East Pittsburgh,Pa., a corporation of Pennsylvania Filed Aug. 18, 1955, Ser. No. 529,267

Claims. (Cl. Z50-213) Our invention relates to radiation imagereproducing devices, and, in particular relates to such devices in whichan output light image is a greatly intensified replica of the intensitydistribution in an incident radiation field. An example of devices ofthis general type is the tube for intensifying X-ray pictures describedin the U.S. Patent 2,523,132 to Mason and Coltman. While the imageintensifier of this patent has proven powerful enough to open a newfield of commerce, there is need for a still more powerful X-ray pictureintensifier; there are also needs for intensifiers of other types ofradiation; and our present invention is applicable to improvements ofradiation detectors of many types.

Briefly stated, our invention employs the properties of an aluminumoxide or similar film when made the cathode surface for an electrondischarge in which the electron current reaching the luminescent `screenof the devices varies from point-to-point in dependence upon thevariations of intensity from point-to-point of a radiation fieldprojected upon the cathode (or grid) surface. Certain modifications ofour invention utilize magnesium oxide cathode surfaces in conjunctionwith photoconductive layers to reproduce radiation images.

One object of our invention is accordingly to provide a novel andsuperior type of radiation-image reproducer.

Another object is to utilize the radiation-responsive properties ofmaterials such as aluminum oxide toreproduce and intensify radiationimages.

Another object is to utilize the variations in electron emission fromaluminum oxide layers and similar substances in response to changes ofincident radiation to control electron flow.

Another object is to utilize the sensitivity of electron emission fromaluminum oxide and similar substances to incident light to produceintensified reproduction of light images.

Another object is to utilize the responsiveness of electron emissionfrom aluminum oxide and similar surfaces to radiation fields to produceelectrical potential distribution over the surfaces which are replicasof the radiation distribution.

Still another object is to combine magnesium oxide layers withphotoconductive layers to produce current flows which reproducevariations of incident radiation.

Yet another object is to provide a novel type of cathode in which amagnesium oxide layer acts as a source of electrons which isself-sustaining for long periods after having received an initialstimulus.

Still another object is to provide a novel type of cathode in whichmagnesium oxide acts as a copious source of secondary electrons whenstimulated by a local source of primary electrons.

Yet another object is to employ the radiation-responsive yproperties ofsecondary-electron ow from the cathode mentioned in the precedingparagraph in an electron-optical system.

The foregoing and other objects of our invention will be apparent uponreading the following description taken in connection with the drawings,in which:

Figure 1 is a diagrammatic view, partly in section, of a tube having aplanar-type cathode comprising a layer of magnesium oxide or aluminumoxide and embodying certain principles of our invention;

Fig. 2 is a similar view of a planar-type cathode illustrating anothermodification of our invention;

Figs. 3 and 4 are similar views of tubes having planartype cathodesuseful in third and fourth modifications of our invention;

Fig. 5 is a schematic view, partly in section, of a radiation-imagereproducer or intensifier embodying another form of our invention; and

Fig. 6 is a similar view of a radiation-image reproducer embodying analuminum oxide or similar cathode in still another form of ourinvention.

Referring in detail to Fig. 1, a planar cathode 1 comprises a glassbacking plate 2 having its surface coated with a layer 3 of conductivematerial such as the conductive glass sold under the trade name NESA,which layer is coated with a layer 4, which may be about 10-4 cms.thick, of magnesium oxide or aluminum oxide.

An electron gun 5 is arranged in ways well known in the art to scan (orood) the layer 4 with an electron beam. A collector electrode 6, whichmay be of screen type, is supported parallel to the surface 4. Theelectrodes 3, 5 and 6 are provided with suitable inleads through acontainer 7 for impressing appropriate potentials between them. When avoltage of 100 to 150 volts is impressed between electrodes 3 and 6 thelayer 4 acts as a cathode furnishing a copious flow of secondary elec.`trons to collector electrode 6 started by the stimulus of primaryelectrons from gun 5. By proper adjustment of the voltage and current toelectrode 6, this discharge can be rendered self-sustaining. However,operation at such voltage and current values that current flow isinterrupted when the electron beam from gun 5 leaves a particular areawill be preferred for some purposes.

An electron phosphor screen 8, e.g., of zinc sulphide may be supportedbehind the collector electrode 6, and if the scanning beam from gun 5 ismodulated with picture signals similar to those of a television system,an image of the picture modulating these signals will appear on screen8.

Fig. 2 shows a modification of the Fig. 1 cathode in which the electrongun 5 is rendered unnecessary by dcoating the magnesium oxide oraluminum oxide layer .i4 with a thin layer 9 ofphotoelectrically-emissive material such as cesium oxide. A thickness of10 to 100 microns would be suitable for most purposes. Light isprojected onto the layer 9 and primary electrons emitted therefromgenerate secondary electrons in the layer 4 causing it to act as acathode. The light projected onto layer 9` may be steady or beinterrupted with results in performance of the cathode similar to thosedescribed in connection with the electron gun 5 in Fig. 1. If thelight-field or other radiation-field projected on layer 9 is an image, areplica thereof of greatly enhanced intensity will appear on anelectron-phosphor screen positioned like screen 8 in Fig. l.

Fig. 3 shows a type of image intensifier tube in which the radiationimage passes through the cathode and controlselectron flow to an outputscreen via a grid. ln this case the radiation image is to be protectedfrom the side of the cathode opposite to the control grid and, to permitsuch projection, the magnesium oxide or aluminum oxide is deposited onthe conductive layer 3 by projecting it through a mask so that a mosaicof separate areas 12 is formed. The control-grid 13 may comprise aforaminated plate in which the perforations are rimmed or lined with amaterial 14 which emits electrons upon Patented Jan. 31, 1961.

incidence of the radiation image which is projected onto it through thecathode 1. The input radiation image, after passing through the cathodeand the electrode 6, produces a charge image on the grid 13 covered bythe photoemissive material 14. In the absence of such a charge image theelectron How from the cathode to electrode 6 is substantially uniformand the grid 13 is biased to such a potential as to reduce or cut offthe flow of electrons to the phosphor screen 8. However, due to theinput radiation, the linings 14 with a potential distributionsubstantially duplicating the input radiation image control the ow ofelectrons from cathode 1, through electron 6 to the electron phosphorscreen 8, producing an intensified replica of the input radiation-imageon the latter. Copending application of W. L. Roberts, Serial No.446,222, filed on July 28, 1954 (Patent 2,871,385, issued January 27,1959, entitled Image Reproduction System) and assigned to the assigneeof this application shows another type of image intensifier employing acontrol grid similar to that just described.

Fig, 4 shows another modification of our invention in which aphotoconductive layer is employed and makes the use of a mosaic cathodestructure unnecessary. Thus, the cathode comprises a layer 15 ofphotoconductive material such as selenium or antimony trisulphide, aboutmicrons thick, sandwiched between a conductive layer 3 and a layer 4 ofmagnesium oxide or aluminum oxide. A source of electrons 5 symbolized inthis case as an electron gun, supplies primary electrons which generatesecondary electrons in layer 4. A pair of meshtype electrodes 17 and 1Sintervene between the cathode assembly 2, 3, 15 and 4 and the thinaluminum layer 19 covering phosphor layer 8. The electrode 17 isimpressed with a constant voltage positive to layer 4, e.g., by 55volts, and the electrode 18 is impressed with a voltage which haspositive rectangular pulses of the general form indicated below thefigure rising e.g. to 50 volts positive, with a trough of e.g. l() voltsnegative, all relative to cathode layer 3.

The mode of operation of Fig. 4 is substantially as follows. Theradiation image which is to be reproduced is focussed through the glassbacking plate 2 and transparent conductive layer 3 on thephotoconductive layer 15. Before the image is so focussed the action ofthe scanning beam (or alternatively a photoelectric layer such as 9 inFig. 2) in bombarding the magnesium oxide layer 4 with electrons causessecondary electron emission therefrom with the result that its surfacerises to nearly 55 volts positive during the positive crest phase of thewave impressed on electrode 18 since the photoconductive layer 15 issubstantially an insulator preventing any replenishing liow from layer 3of electrons to replace the secondaries attracted to electrode 17.During the negative phase of the voltage on electrode 18, thc potentialof the surface of layer 4 remains nearly at the same potential becauseof this isolating action of layer 15, and this situation continues forall elemental areas of the screen so long as they remain dark after theradiation image is projected. However, in those picture areas where theradiation intensity of the projected irnage is substantial, thephotoconductive layer 15 becomes substantially conductive and permitselectrons to flow through it from conductive layer 3 to magnesium oxidelayer 4, and the potential at the surface of the latter fallssubstantially below the 55 volt positive potential of electrode 17.Thus, a potential image is formed on the surface of layer 4 which is areplica of the intensity distribution of the radiation image, andelectrons attracted from the higher intensity areas when electrode 18again becomes positive will have energy enough to pass through themeshes of electrode 1,7 to phosphor screen 3, while electrons from thedarker areas of the images, since they emanate from areas of layer 4over 55 volts in positive potential, will not have sufficient energy topass ciccl trode 18. A light image duplicating the radiation image indistribution will thus appear at phosphor layer 8.

Fig. 5 shows a modification of our invention which comprises a series ofsuperposed layers without intervening spaces. Thus, a glass plate 2,which may, or may not, be a portion of a vacuum-tight container,supports a conductive transparent layer 3 covered by a layer 1S ofphotoconductive material, which is coated in turn by a layer 4 ofmagnesium oxide and on this layer is deposited a thin layer 21 ofaluminum or other suitable conductor, and a layer 8 of an electronphosphor such as zinc sulphide is deposited on the latter. When aradiation field to which the photoconductive layer 15 is susceptible isprojected onto it through glass plate 2, the areas of the photoconductor15 where the radiation eld is of high intensity lose their insulatingpower and the entire voltage impressed between the conductive layers 3and 21 is concentrated across the magnesium oxide layer 4 thus causing acopious ow of electrons through it into impact with phosphor screen 8.On the other hand, those areas of photoconductive layer 15 where theradiation field is dark retain high insulating power so that the voltagegradient in the magnesium oxide layer 4 directly beneath them is low andalmost no electrons are projected into impact with phosphor screen 8 atdark image areas. A light image, duplicating in distribution theradiation eld, thus appears on output screen 8.

While we have shown walls 7 enclosing the Fig. 5 layers, it may bedesirable for some uses to omit these, and this may be done since thematerials forming the layers are chemically inert to air.

Fig. 6 shows a form of our invention which makes use of the propertyshown by aluminum oxide and magnesium oxide of varying their emission ofsecondary electrons when the intensity of incident radiation on themchanges. Thus, a glass backing plate 2 carries a layer of transparentconductive material 3, such as NESA, on which is deposited a layer 4 ofaluminum oxide or magnesium oxide. The latter is coated in turn with alayer 9 of cesium oxide or other photoelectrically-emissive material. Afirst grid 22 and a second grid 23 are positioned parallel to the layer9 and to phosphor layer 8 covered by a thin layer 21 of aluminum. Thefirst grid is impressed with a potential V1 relative to conductive layer3 and when the photoemissive layer is irradiated or bombarded withprimary electrons of suitable energies, secondary electrons are drawnfrom the composite cathode structure to electrode 22 until the oxidelayer 4 rises nearly to the potential V1 of the latter so long as noradiation is projected onto the layer 4 through glass plate 2. Since theoxide layer is normally of very high resistivity and very thin, a highvoltage gradient tends to build up in it and this results in anavalanching of electrons which permits electrons to ow to grid 22 undersubstantially space-charge limited conditions. When, however, anyelemental area of layer 4 is irradiated by such radiation the secondaryemissivity of that elemental area of layer 4 decreases probably due tothe fact that the oxide layer becomes more conducting, and consequently,the potential of said area becomes less positive. The surface of oxidelayer 4 thus becomes a cathode with a potential distribution varyingfrom point to point corresponding to the radiation image projectedthrough glass plate 2. The grid 23 is given a potential intermediatebetween the maximum and minmum values present in this potentialdistribution. As a result, electrons emitted from elemental areas of thecathode where the radiation intensity is high will be accelerated toenergies which enable them to pass through grid 23 and into incidencewith phosphor layer 8, while electrons from radiation areas of lowerintensity will be unable to pass electrode 23. A light image having apattern like the radiation image thus appears on the output screen 8.

We claim as our invention:

1. An image display device comprising a large area electron source, anoutput screen on which the electrons from said source are projected andgrid means positioned between said source and said output screen, saidelectron source comprising a first layer of electrical conductivematerial and la second layer deposited on said first layer on the sidethereof facing said output screen of a material selected from the groupconsisting of magnesium oxide and aluminum oxide, means for impressing auniform charge on the surface of said second layer to establish a highvoltage gradient within said second layer to cause field emission ofelectrons from said source, means for projecting a radiation image ontosaid source to establish va charge pattern on the surface of said secondlayer corresponding to said light image and means for accelerating anelectron image corresponding to said charge pattern from said source tosaid output screen, said grid means comprising a first grid and a secondgrid, said first grid disposed nearer to said electron source than saidsecond grid and having a potential applied thereto more positive thanthat applied to said electron source, said second grid having apotential yapplied thereto between the potential applied to said rstgrid and the potential assumed by said second `layer under substantialradiation.

2. An image display device comprising a large area electron source, anoutput screen on which the electrons from said source are projected landgrid means positioned between said source land said output screen, saidelectron source comprising a first layer of electrical conductivematerial and a second layer deposited on said first layer on the sidethereof facing said output screen of a material selected from the groupconsisting of magnesium oxide and aluminum oxide, means for impressing auniform charge on the surface of -said second layer to establish a highvoltage gradient within said second layer to cause field emission ofelectrons onto said source, means for projecting a `radiation image ontosaid source to establish a charge pattern on the surface of said secondlayer corresponding to said light image and means for accelerating anelectron image corresponding to said charge pattern from said source tosaid output screen, said means for impressing a uniform voltage on thesurface ofv said second layer comprising a layer ofphotoelectrically-emissive material superimposed on said second layer.

3. An image display device comprising a large area electron source, anoutput screen on which the electrons from said electron source areprojected, said electron source comprising a layer of electricalconductive material and a layer of a material exhibiting the property ofelectron emission in response to an electric field across said layer;means for establishing a field across said electron emissive layer ofsaid electron source comprising a coating of a photoelectric materialprovided on the exposed surface of said electron emissive layer and anauxiliary radiation source for directing radiations onto saidphotoelectric coating, a first grid member positioned between saidelectron source and said output screen and means for providing apotential on said first grid member positive with respect to thepotential applied to said electrical conductive member such that thesurface o-f said second layer of said electron source tends to seek thepotential of said first grid member thereby establishing a field acrosssaid electron emissive layer such that said electron emissive layer willemit electrons, a second grid positioned between said first grid andsaid output screen and means for providing potential difference betweensaid second -grid and said output screen to direct the electrons fromsaid electron source into incidence with said output screen.

4. An image display device comprising an electron source, an outputscreen of a material which emits light in response to electronbombardment projected from said electron source, said electron sourcecomprising a layer of electrical conductive material and an electronemissive layer of a material selected from a group consisting ofmagnesium oxide and aluminum oxide, means for establishing a uniformcharge on the surface of said electron emissive layer to establish anelectric field across said electron emissive layer to cause fieldemission of electrons from said source, said charge establishing meanscomprising an auxiliary electron source for directing elec'- trons ontothe surface of said electron emissive layer and a iirst grid memberpositioned between said electron source and said output screen andmaintained at a positive potential with respect to the potential appliedto said electrical conductive layer of said electron source, a secondgrid positioned between said first grid and said output screen and meansfor providing a pulsating voltage on said second grid and meansproviding a positive potential on said output screen with respect tosaid electron source to accelerate electrons passing through said secondgrid into incidence with said output screen.

5, An image translation device comprising a large area electron source,an output screen which emits light in response to electron bombardmentfrom said electron source, said electron source comprising a layer of anelectrical conductive material and a layer of a material that exhibitsthe property of emission of electrons in response to an electric fieldestablished Within said electron emissive layer, means for establishingan electric field in said electron emissive layer, said fieldestablishing mea-ns comprising a first grid member positioned betweensaid output screen and said electron source, a potential source forproviding a positive potential on said first grid with respect to saidelectrical conductive layer, and an auxiliary electron source fordirecting electrons onto the surface of said electron emissive layer, asecond grid positioned between said first grid and said output screen,said second grid provided with a radiation sensitive coating responsiveto an input radiation image to provide a charge image thereoncorresponding to said radiation image and means for accelerating theelectrons from said electron emissive layer modulated by the chargeimage on said second grid into incidence with said output screen.

References Cited in the file of this patent UNITED STATES PATENTS2,058,941 Arnhym Oct. 27, 1936 2,199,438 Lubszynski May 7, 19402,603,757 Sheldon July 15, 1952 2,699,511 Sheldon Jan. 11, 19552,708,726 Atherton May 17, 1955 OTHER REFERENCES Malter: Thin Film FieldEmission, Physical Review, vol. 50, page 48.

